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306 PROCEEDINGS OF THE 29TH TURBOMACHINERY SYMPOSIUM As Franz had no opportunity to design individual engine components, a decision was made to design an experimental engine, the 004A, which would be thermodynamically and aerody- namically similar to the final production engine. The goal in developing the 004A was to have an operating engine in the shortest timeframe without consideration for engine weight, manufacturing considerations, or minimizing the use of strategic materials. Based on the results of the 004A engine, the production 004B engine was to be built. The compressor utilized pure reaction blading that resulted in a pressure ratio of 3.14:1 in eight compression stages. The engine airflow rate was 46.6 lb/sec (21.2 kg/sec). The turbine was based on steam turbine experience of AEG, Berlin, and blades were not of the vortex design as proposed by Whittle. Franz recognized the superiority of an annular combustor design but opted for a six-can type combustor, as he knew that these would present less of a problem and permit bench testing with a single can. On July 18, the first flight of the ME 262 powered by two Jumo 004 jets took place and lasted for 12 minutes. The ME 262 is shown in Figure 69. Details on the ME-262 may be found in Morgan (1994). Figure 69. Messerschmitt ME-262 Fighter, the World’s First Operational Jet Fighter Powered by Two Junkers Jumo 004B Turbojets. (The aircraft was capable of 550 mph and became operational in 1944.) (Meher-Homji, 1996; Courtesy ASME) Development of the 004 B Production Engine Based on the excellent flight results, the air ministry issued a contract for 80 engines. These engines, rated at a thrust of 1850 lb, were used for further engine development and airframe testing. The 004A engine was unsuitable for production because of its considerable weight and its high utilization of strategic materials (Ni, Co, Molybdenum), which were not available to Germany at that time. Because of this, the 004B engine was designed to use a minimum amount of strategic materials. All the hot metal parts including the combustion chamber were changed to mild steel (SAE 1010) and were protected against oxidation by aluminum coating. The later version of the 004B engine had hollow air-cooled stator vanes. Compressor discharge air was used to cool the blades. With the hollow Cromandur sheet-metal blade, the complete 004B engine had less that 5 lb of chromium. A cutaway view of the Junkers Jumo 004 engine is shown in Figure 70. Figure 70. Cross Section of the Junkers Jumo 004 Turbojet Showing Eight-Stage Axial Compressor, Six-Can Annular Combustors, and Single-Stage Air-Cooled Turbine. (Neville and Silsbee, 1948) Turbine Blade Failures During the summer of 1943, several turbine blade failures were experienced due to a sixth order excitation (6 number of combustors) when operating at full speed. The Junkers team worked diligently to resolve the problems. Franz recalls that he used the unconventional method to determine blade natural frequency by asking a professional musician to stroke the blades with a violin bow and then use his trained musical ear to determine the ringing natural frequency. The Air Ministry was, however, getting increasingly impatient and scheduled a conference in December 1943 at the Junkers Dessau plant, to be attended by turbine experts from government, industry, and academia. Max Bentele, who was instrumental in solving the problem, attended this conference and listened to the numerous arguments pertaining to material defects, grain size, and manufacturing tolerances. As recounted by Bentele in his autobiography (Bentele, 1991), these were only secondary factors. When his turn came, he stated clearly to the assembled group the underlying cause of the problem, namely that the six combustor cans and the three struts of the jet nozzle housing after the turbine were the culprits. These induced forced excitation on the turbine rotor blades where a sixth order resonance occurred with the blade bending frequency in the upper speed range. The predominance of the sixth order excitation was due to the six combustor cans (undisturbed by the 36 nozzles) and the second harmonic of the three struts downstream of the rotor. In the 004A engine, this resonance was above the operating speed range, but in the 004B it had slipped because of the slightly higher turbine speed and due to the higher turbine temperatures. The problem was solved by increasing the blade natural frequency by increasing blade taper, shortening blades by 1 mm (.039 inch), and reducing the operating speed of the engine from 9000 to 8700 rpm. TURBOJET DEVELOPMENT IN THE USA The Whittle Engine in the USA Upon declaration of World War II, Sir Henry Tizard, who was Chairman of the British Aeronautical Research Council, proposed sharing jet technology with the United States and started official talks. US military intelligence had, however, been filing reports about jet propulsion work in both England and Germany, and Major General Hap Arnold visited Britain to examine this technology. In May 1941, Arnold put in a formal request for jet technology. On July 21, 1941, Roxby Cox and Roy Shoults of GE visited the Power Jets Limited and the Gloster factory. A decision was made to mass-produce this engine in the US and GE was chosen to build the engine. As reported by Ford (1992), a GE delegation visited Washington on September 4, and was handed a sheaf of drawings with Hap Arnold stating, “Gentlemen, I give you the Whittle Engine.” GE committed to build a working engine within six months. Bell Aircraft was commissioned to build a prototype jet fighter. On October 1, 1941, the Whittle W.1X was flown to the US in a B-24 bomber and made its way to Building 34 North at the GE Lynn, Massachusetts, facility. On October 16, the W.1X was fired up. In a remarkable engineering effort, the GE team made some modifications to the design and within six months ran an engine on March 18, 1942. Later, Whittle visited Boston to help solve a problem with burning bearings. In August, GE delivered two engines (designated the I-A) to Bell Aircraft and the first flight of the Bell P-59 was made on October 1, 1942, exactly one year after the W.1X left Britain. An excellent description of the initial US jet engine work is made by Ford (1992). Engine Development at Westinghouse While most attention is traditionally focused on British and German turbojet development, it should be noted that a remarkable achievement was accomplished by a design group at Westinghouse led by Reinout P. Kroon. As a result of Kroon’s visit to the Navy BureauPDF Image | THE HISTORICAL EVOLUTION OF TURBOMACHINERY
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